The invention relates to a thermally insulated cell for the electrolytic purification of aluminum comprising a trough with a steel shell which is surrounded by an outer wall, is lined with a material which can resist high temperatures and electrolyte and is closed off by a lid; an electrolyte based on alkali chlorides; segregation wells for the addition of the aluminum which is to be purified and for the precipitation of crystalline components; a supply means which is used to feed-in electrolyte material and is also conceived as a waste gas extractor; and a collection and run-off system for the high purity aluminum.
The electrolytic purification of aluminum is based on the fact that components which are less noble than aluminum (e.g. sodium, potassium, lithium, magnesium, calcium) and are present in the alloy to be treated dissolve anodically in the aluminum but cannot be precipitated at the cathode and the components which are more noble than aluminum do not dissolve anodically and thus form crystals and remain in the anode metal. The three layer cell for electrolytic purification of aluminum has been known since the start of this century and contains three molten layers. The lowest, heavy layer is normally an Al-Cu-Si-Fe alloy the surface of which is at the same time the anode. The electrolyte layer comprising fluorides and/or chlorides of alkali and alkaline earth metals. The purified aluminum which is the third uppermost layer, the lower surface of which forms the cathode.
By applying a direct current to that cell the aluminum is oxidized to trivalent aluminum ions. These ions move through the electrolyte to the cathode where they are again reduced to metallic aluminum.
By means of a segregation well which is operated at a temperature lower than the normally 700.degree.-800.degree. C. of the purification cell, impurities which crystallize out, in particular as intermetallic phases containing Al, Cu, Fe and Si, are removed.
The energy consumed in the conventional three layer purification cell is relatively high. Typical values for the cell voltage are ca. 5.5 V with a current yield of about 78-97%. This results in an energy consumption of appproximately 16-18 kwh/kg of purified aluminum.
From a physics standpoint two possible developments could lead to a reduction of the energy consumption:
(1) The use of electrolytes with higher electrical conductivity and/or PA1 (2) reducing the interpolar distance i.e. the thickness of the electrolyte layer. PA1 (1) A horizontal arrangement in which the electrode units are installed vertical or almost vertical, PA1 (2) a vertical arrangement with horizontal or slightly inclined electrode units.
The electrolyte layer, which is normally 10-15 mm thick in the conventional three layer process, can not be reduced at will without risking the possibility of mechanical contamination of the layer of purified aluminum by contact with the anodically polarized aluminum. Recently vessel-shaped or vertically displaceable diaphragms have been employed to help reduce the high energy consumption of the cell.
The object of the present invention is to develop for the electrolytic purification of aluminum, a cell which not only features low energy consumption but also high metallurgical efficiency and can be realized with low investment costs.